This invention relates to switched reluctance (SRM) motors and, more particularly, to a current decay control circuit for such motors.
Switched reluctance, or SRM motors are well-known in the art. One problem with operating these motors is noise caused by the recovery of current in the motor phase windings as each phase is switched at the end of its cycle. It will be understood that the current representing the energy input of a particular phase is supplied to the phase windings during that phase's active portion of a switching cycle. As the motor is switched from one phase to another, the residual energy in the deactivated winding decays off. This energy typically represents approximately thirty percent (30% ) of the energy supplied to the phase winding during its active period and is referred to as the "tail decay energy". Since the phase winding is an inductive element, it attempts to maintain the current flow through the winding; even though the energy must be substantially dissipated before the winding is re-energized during the next phase cycle. Accordingly, the decay must be a rapid decay. One effect of this energy reduction is the ringing effect which is caused at the transition between the active and inactive portions of the phase current curve. This can be seen as the abrupt transition in slope in the current curve between the shallow slope representing the active portion of the cycle and the steep slope where the current is driving to zero when the phase becomes inactive. The result of this ringing or transfer of forces into the motor frame causes noise, and this noise is on the order of 50 dBa.
Commutation circuits are used to control switching between motor phases as a function of various motor operating parameters. Such circuits typically employ a pulse width modulator (PWM). PWM circuits, in addition to controlling the application of voltage to the motor phases can also be used to control the residual current decay. These circuits operate to control this decay in accordance with a defined algorithm. However, it is a drawback of these decay control circuits that they use a conventional 100% forced commutation decay; and, as such, they tend to aggravate the noise problem. One attempt at decay is suggested by C. Y. Wu and C. Pollock in their paper Analysis and Reduction of Vibration and Acoustic Noise in the Switched Reluctance Drive; (IEEE Proceedings, Industrial applications Section, 28th, Annual Meeting, October 1993). The approach described in this paper involves a zero voltage decay of the current in a phase winding, when the phase is switched "off", over a period equal to one-half the resonant time period of the motor, and with a subsequent forced commutation of the remainder of the "off" time. The drawback with this approach is that there is but one decay interval divided into two segments. As a result, the degree of control over the slope of the curve as the current is driven to zero is not as flexible in significantly reducing the noise.
While the above approach may be effective, there are nonetheless other approaches which may be more effective to facilitate tail decay while reducing noise.